Prior microwave methods for cardivascular monitoring are based on heart rate detection from Doppler (velocity) processed data in back scatter or narrow angle bistatic geometries. No hemodynamic measures are available. Important improvements in microstrip antenna technologies in the instant invention ameliorate loss of signal with subject movement and interference from unrelated movement or external signal sources that effect sensor operation by way of leakage or fringing fields.
The instant invention for a microwave monitor of cardiac hemodymanics is distinguished from ultrasound systems since it is neither labor intensive nor greatly dependant upon operator skill. These are benefits of the fact that transducer placement is not critical and it is not an imaging method. It is distinguished from radio-isotope methods in that neither ionizing radiation nor isotopes are involved and chamber volumes are estimated by changes in forward scattered complex field amplitudes, especially the phase of S.sub.21. The instant invention is distinguished from other microwave methods in its use of vector processing of coherent forward scattered fields; by the availability of, and processing for, hemodynamic indices rather than simply heart rate; and by antenna technologies that reduce artifact from slippage, leakage and fringing field effects. This ease of use and relatively noncritical antenna positioning combine to produce a robust method for ambulatory monitoring of cardiac hemodynamics that is not available with alternative technologies.
There is no prior art in microwave hemodynamic measurements in humans that uses flexible, closely coupled microstrip antennas and vector processing of the scattered complex field amplitudes.
Past methods used were based on waveguide antennas and scalar processing of forward scattered fields. Past methods, therefore, are insensitive to electrical length(s) of the propagation path and unsuitable for ambulatory use.
Human work is limited to Doppler processing of back scattered or narrow angle bistatic systems. Doppler processing detects the velocity of interfacial movement using a noncontacting wave guide antenna and monostatic Doppler processing of the back scattered field. It does not measure the complex field amplitudes (CFA). Furthermore, there is little or no penetration of the sensor signal beyond the first interface because of the high reflection coefficient from air to chest wall and high carrier frequency (ca 10 GHz) as opposed to the UHF band used by the instant invention needed to allow extraction of Doppler components from the slowly moving chest wall.
Recent heart rate monitors explored several frequencies between 2 and 12 GHz with patch antennas and Doppler processing from monastatic (back scattered) or narrow angle bistatic configurations. The antennas were not in skin contact, but rather placed on top of protective clothing.
Later, two frequencies (X band and S band) were studied in a fully shielded enclosure except for a thick Teflon face on one side. Doppler processing was replaced by detection of oscillator pulling by changes in collector current with load mismatch; but results were unreliable due to signal disruption and nearby, unrelated movement provented successful operation of the sensor. Also, related movement such as that involving the verlying muscle was still more troublesome and the signal was disrupted to the point of becoming unusable. The system was limited to the detection of arterial pulse with later processing for lost beats.
A constant problem with prior designs derives from movement artifact. This has two forms; (1) transducer movement with respect to the target issue whether due to surface slippage, inertial effects, or movement of internal structures; and (2) related movement such as changes in cable capacitance with bending or leakage/fringe fields leading to interference. The design procedures of prior art did not provide for low profile patch antennas that are closely coupled and conformal. The sensor must be mechanically coupled to the target with a low moment of inertia. Prior art placement of the sensor increased the effective moment of inertia and led to an inability to accomplish constant, close coupling of the sensor to the target.
Additionally, prior techniques failed to provide close electromagnetic coupling because of the thick radome. A thin, conformal radome also decreases leakage effects and fringing fields.
No prior art utilized vector analysis of scattering parmeter S.sub.21 which provides the advantages of phase information, the advantage of larger radar cross section, and the advantage of contacting microstrip antennas. Furthermore, none of the prior art is suitable for application to ambulatory monitoring.